In the NMR of strong resonances such as water, the phenomenon of radiation damping has long been considered a nuisance. When a large net magnetization is present, radiation damping causes the magnetization vector to nutate back towards the +z axis (parallel to the magnetic field) at a rate proportional to the magnitude of the transverse magnetization. This description of radiation damping leads to the conclusion that there will be no damping effect when there is no net magnetization in the transverse plane. Of particular interest here, this implies that magnetization can be stored purely along -z. In practice, however, it is observed that even a ~perfect~ c pulse (a c pulse followed by a gradient to dephase all transverse magnetization) leads to observable magnetization as the magnetization vector nutates from -z through the transverse plane to +z under the influence of radiation damping. In the presence of radiation damping, magnetization left purely along -z constitutes an unstable state since the radiation damping mechanism provides a restoring force that tends to nutate magnetization back toward +z. While the restoring force is in fact are along -z, any slight perturbation can result in a transverse magnetization component that grows exponentially at first, giving rise to the signals observed in experiments. We have found that the residual RF leakage from the spectrometer and even thermal noise for the RF coil produce perturbing fields sufficient to initiate radiation damping. To avoid radiation damping it is necessary to continuously suppress the initiation of radiation damping.
Showing the most recent 10 out of 12 publications
